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ASTR 2310: Chapter 6

Astronomical Detection of Light

The Telescope as a Camera

Refraction and Reflection Telescopes

Quality of Images

Astronomical Instruments and Detectors

Observations and Photon Counting

Other Wavelengths

Modern Telescopes
Refracting / Reflecting Telescopes
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Refracting
Telescope:
Lens focuses
light onto the
focal plane
Focal length
Focal length
Reflecting
Telescope:
Concave Mirror
focuses light
onto the focal
plane
Almost all modern telescopes are reflecting telescopes.
Secondary Optics
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In reflecting
telescopes:
Secondary
mirror, to redirect light path
towards back or
side of
incoming light
path.
Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics.
Disadvantages of
Refracting Telescopes
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• Chromatic aberration:
Different wavelengths are
focused at different focal
lengths (prism effect).
Can be corrected, but not
eliminated by second lens
out of different material.
• Difficult and expensive
to produce: All surfaces
must be perfectly
shaped; glass must be
flawless; lens can only be
supported at the edges.
The Best Location for a Telescope
0
Far away from civilization – to avoid light pollution
The Best Location for a Telescope (II)
0
Paranal Observatory (ESO), Chile
http://en.wikipedia.org/wiki/Paranal_Observatory
On high mountain-tops – to avoid atmospheric
turbulence ( seeing) and other weather effects
The Powers of a Telescope:
Size does matter!
1. Light-gathering
power: Depends on the
surface area A of the
primary lens / mirror,
proportional to
diameter squared:
Θ = π (D/2)2
D
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The Powers of a Telescope (II)
2. Resolving power: Wave nature of light
=> The telescope aperture produces fringe
rings that set a limit to the resolution of the
telescope.
Astronomers can’t eliminate these
diffraction fringes, but the larger a telescope
is in diameter, the smaller the diffraction
fringes are. Thus the larger the telescope,
the better its resolving power.
 min = 1.22 ( /D) (radians)
For optical wavelengths, this gives
 min = 11.6 arcsec / D[cm]
min
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The Powers of a Telescope (III)
3. Magnifying Power = ability of the telescope
to make the image appear bigger.
A larger magnification does not improve the
resolving power of the telescope!
Traditional Telescopes (I)
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Secondary mirror
Traditional primary
mirror: sturdy, heavy
to avoid distortions.
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Traditional
Telescopes (II)
The 4-m
Mayall
Telescope at
Kitt Peak
National
Observatory
(Arizona)
Astronomical Telescopes
0
Often very large to gather
large amounts of light.
In order to observe
forms of radiation other
than visible light, very
different telescope
designs are needed.
The northern Gemini Telescope on Hawaii
Examples of Modern
Telescope Design
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
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0
Seeing
Weather
conditions and
turbulence in the
atmosphere set
further limits to
the quality of
astronomical
images
Bad seeing
Good seeing
Advances in Modern Telescope Design
Lighter mirrors with lighter support structures,
to be controlled dynamically by computers
Floppy mirror
Segmented mirror
Adaptive Optics
0
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
Interferometry
0
Recall: Resolving power of a telescope depends on diameter D.
 Combine the signals from
several smaller telescopes to
simulate one big mirror 
Interferometry
CCD Imaging
0
CCD = Charge-coupled device
• More sensitive than
photographic plates
• Data can be read directly
into computer memory,
allowing easy electronic
manipulations
False-color image to visualize
brightness contours
The Spectrograph
0
Using a prism (or a grating), light can
be split up into different wavelengths
(colors!) to produce a spectrum.
Spectral lines in a
spectrum tell us about the
chemical composition and
other properties of the
observed object
Radio Astronomy
0
Recall: Radio waves of  ~ 1 cm – 1 m also penetrate the
Earth’s atmosphere and can be observed from the ground.
Radio Telescopes
0
Large dish focuses
the energy of radio
waves onto a small
receiver (antenna)
Amplified signals are
stored in computers
and converted into
images, spectra, etc.
Radio Interferometry
0
Just as for optical
telescopes, the
resolving power of a
radio telescope
depends on the
diameter of the objective
lens or mirror min =
1.22 /D.
For radio telescopes,
this is a big problem:
Radio waves are much
longer than visible light
 Use interferometry to
The Very Large Array (VLA): 27
dishes are combined to simulate a
large dish of 36 km in diameter.
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The Largest Radio Telescopes
The 100-m Green Bank
Telescope in Green Bank, West
The 300-m telescope in
Science of Radio Astronomy
Radio astronomy reveals several features,
not visible at other wavelengths:
• Neutral hydrogen clouds (which don’t
emit any visible light), containing ~ 90 %
of all the atoms in the universe.
• Molecules (often located in dense
clouds, where visible light is
completely absorbed).
• Radio waves penetrate gas and
dust clouds, so we can observe
regions from which visible light is
heavily absorbed.
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Infrared Astronomy
0
Most infrared radiation is absorbed in the lower atmosphere.
However, from
high mountain
tops or highflying aircraft,
some infrared
radiation can
still be
observed.
NASA infrared telescope on Mauna Kea, Hawaii
Infrared Telescopes
Spitzer Space Telescope
WIRO 2.3m
Ultraviolet Astronomy
• Ultraviolet radiation with  < 290 nm is
completely absorbed in the ozone layer
of the atmosphere.
• Ultraviolet astronomy has to be done
from satellites.
• Several successful ultraviolet astronomy
satellites: IUE, EUVE, FUSE
• Ultraviolet radiation traces hot (tens of
thousands of degrees), moderately
ionized gas in the universe.
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NASA’s Great Observatories in Space (I)
The Hubble Space Telescope
• Launched in 1990;
maintained and
upgraded by several
space shuttle service
missions throughout the
1990s and early 2000’s
• Avoids turbulence
in Earth’s
atmosphere
• Extends imaging
and spectroscopy to
(invisible) infrared
and ultraviolet
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Hubble Space Telescope Images
Mars with its
polar ice cap
Nebula around
A dust-filled galaxy
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NASA’s Great Observatories in Space (II)
The Compton Gamma-Ray Observatory
Operated from
1991 to 2000
Observation of
high-energy
gamma-ray
emission, tracing
the most violent
processes in the
universe.
NASA’s Great Observatories in Space (III)
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The Chandra X-ray Telescope
Launched in 1999 into a highly
eccentric orbit that takes it 1/3
of the way to the moon!
X-rays trace hot (million
degrees), highly ionized
gas in the universe.
Two colliding
galaxies,
triggering a
burst of star
formation
Very hot gas
in a cluster
of galaxies
Saturn
Chandra X-ray Observatory
Shuttle launched, highly eccentric orbit.
The Highest Tech Mirrors
Ever!
• Chandra is the first X-ray telescope to
have image as sharp as optical
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NASA’s Great Observatories in Space (IV)
The Spitzer Space Telescope
Launched in 2003
Infrared light traces warm
dust in the universe.
The detector needs to be
cooled to -273 oC (-459 oF).
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Spitzer Space Telescope Images
A Comet
Warm dust in a
young spiral galaxy
Newborn stars that
would be hidden
from our view in
Spitzer Space Telescope
• Discovered by a Wyoming grad student and
professor. The “Cowboy Cluster” – a new Globular
Kepler’s Supernova with all
three of NASA’s Great
Observatories
• Just 400 years ago:
(Oct. 9, 1604)
• Then a bright, naked eye
object (no telescopes)
• It’s still blowing up – now 14
light years wide and
expanding at 4 million mph.
• There’s material there at
MANY temperatures, so
many wavelengths are
needed to understand it.
A Multiwavelength Look at Cygnus A
• A merger-product, and powerful radio galaxy.
The Future of Space-Based
Optical/Infrared Astronomy:
The James Webb Space Telescope